INVESTIGATING PULSE MORPHOLOGY IN GX-1+4

Observational and theoretical evidence points to the existence of an u nusually high magnetic field on GX 1 + 4. The pulsar is thus an ideal laboratory for studying two-photon cyclotron emission, an important so urce of photons of frequency significantly less than the cyclotron fre quency in X-ray pulsars. Low-frequency approximations to the two-photo n cyclotron emission transition probabilities are derived. These are u sed to calculate the theoretical opening angle of the double-humped pu lse shape predicted by the two-photon cyclotron emission model. The th eoretical pulse shape, incorporating the effects of gravitational ligh t bending, is compared with observations of GX 1 + 4. Observed light c urves have opening angles consistent with the theoretically predicted maximum value.